Diapycnal and along isopycnal mixing, estimated from the tracer release experiment, at the North Atlantic oxygen minimum zone

A deliberate Guinea Upwelling Tracer Release Experiment (GUTRE) in 2008 - 2010 was used to study diapycnal mixing and lateral stirring in the thermocline of tropical northeastern Atlantic. The ultimate goal of this work was to investigate oxygen supply pathways at the upper boundary of North Atlantic oxygen minimum zone. The mean diapycnal mixing coefficients were computed for a tracer patch integrated over 30 months. The strong variation of stratification in the tracer occupied area offered the advantage of using the advection-diffusion equation in isopycnal coordinates with the thickness-weighted averaging, when analyzing the diapycnal tracer spread. Overall, the vertical mixing estimates were found about 30% smaller than estimates in North Atlantic Tracer Release Experiment (NATRE), performed about 15° to the north from our experiment. In general, the internal wave-wave interaction models predict reduced mixing from the breaking of internal waves at low latitudes. Thus, GUTRE results are larger than parametrized by the low latitude of the site (4°N - 12°N). The mean isopycnal mixing coefficient was estimated by computing the growth of the second moment of a cloud of tracer. The zonal component of lateral mixing was found more than two times larger than the meridional component. The finding reflects the stretching effect of zonal jets prevalent in the tropical Atlantic Ocean. The application of conceptual jet model allowed to evaluate the mean zonal jet velocities which cause an enhancement of mixing in zonal direction. Additionally, the effective mixing coefficient was computed which indicated that zonal jets do not merely stretch the tracer patch in zonal directions, but increase mixing by increasing tracer gradients. The uncertainties of the estimates were inferred from synthetic particle release using a high resolution ocean circulation model. Finally, a large database of vertical oxygen profiles in oxygen minimum region and high precision diapycnal mixing estimates allowed for a good estimate of diapycnal oxygen supply through the upper boundary into the oxygen minimum zone. In comparison to conceptual model of oxygen consumption with depth, the diapycnal oxygen supply was estimated to be as large as about half of the lateral oxygen supply. Furthermore, in the oxygen minimum region, the separate shallow oxygen minimum was found at about 100 m depth. The finding indicates that there is no direct net vertical oxygen flux from the surface layer into the oxygen minimum zone. All of oxygen supply, hence, is associated with remote pathways

An acoustic view of ocean mixing

Knowledge of the parameter K (turbulent diffusivity/"mixing intensity") is a key to understand transport processes of matter and energy in the ocean. Especially the almost vertical component of K across the ocean stratification (diapycnal diffusivity) is vital for research on biogeochemical cycles or greenhouse gas budgets. Recent boost in precision of water velocity data that can be obtained from vessel-mounted acoustic instruments (vmADCP) allows identifying ocean regions of elevated diapycnal diffusivity during research cruises - in high horizontal resolution and without extra ship time needed. This contribution relates acoustic data from two cruises in the Tropical North East Atlantic Oxygen Minimum Zone to simultaneous field observations of diapycnal diffusivity: pointwise measurements by a microstructure profiler as well as one integrative value from a large scale Tracer Release Experiment

Diapycnal oxygen supply to the tropical North Atlantic oxygen minimum zone

The replenishment of consumed oxygen in the open ocean oxygen minimum zone (OMZ) off northwest Africa is accomplished by oxygen transport across and along density surfaces, i.e. diapycnal and isopycnal oxygen supply. Here the diapycnal oxygen supply is investigated using a large observational set of oxygen profiles and diapycnal mixing data from years 2008 to 2010. Diapycnal mixing is inferred from different sources: (i) a large-scale tracer release experiment, (ii) microstructure profiles, and (iii) shipboard acoustic current measurements plus density profiles. From these measurements, the average diapycnal diffusivity in the studied depth interval from 150 to 500m is estimated to be 1×10−5 m2 s−1, with lower and upper 95%confidence limits of 0.8×10−5 m2 s−1 and 1.4×10−5 m2 s−1. Diapycnal diffusivity in this depth range is predominantly caused by turbulence, and shows no significant vertical gradient. Diapycnal mixing is found to contribute substantially to the oxygen supply of the OMZ. Within the OMZ core, 1.5 μmol kg−1 yr−1 of oxygen is supplied via diapycnal mixing, contributing about one-third of the total demand. This oxygen which is supplied via diapycnal mixing originates from oxygen that has been laterally supplied within the upper CentralWater layer above the OMZ, and within the Antarctic Intermediate Water layer below the OMZ. Due to the existence of a separate shallow oxygen minimum at about 100m depth throughout most of the study area, there is no net vertical oxygen flux from the surface layer into the Central Water layer. Thus all oxygen supply of the OMZ is associated with remote pathways

Diapycnal diffusivity at the upper boundary of the tropical North Atlantic oxygen minimum zone

A deliberate tracer release experiment in 2008–2010 was used to study diapycnal mixing in the tropical northeastern Atlantic. The tracer (CF3SF5) was injected on the isopycnal surface σΘ = 26.88 kg m−3, which corresponds to about 330 m depth. Three surveys, performed 7, 20, and 30 months after the release, sampled the vertically and laterally expanding tracer patch. The mean diapycnal mixing estimate over the entire region occupied by the tracer and the period of 30 months was found to be (1.19 ± 0.18) × 10−5 m2 s−1, or, alternatively, (3.07 ± 0.58) × 10−11 (kg m−3)2 s−1 as computed from the advection-diffusion equation in isopycnal coordinates with the thickness-weighted averaging. The latter method is preferable in the regions of different stratification for it yields local diapycnal mixing estimates varying less with stratification than their Cartesian coordinate counterparts. Results of this study are comparable to the results of the North Atlantic tracer release experiment (NATRE). However, the internal wave-wave interaction models predict reduced mixing from the breaking of internal waves at low latitudes. Thus, the diapycnal diffusivity found in this study is higher than parameterized by the low latitude of the site (4°N–12°N)

Tracer Survey in the Cape Verde Region Traceraufnahme in der Kapverdenregion Cruise No. 10, Leg 1 October 31 – December 06, 2008 Ponta Delgada (Portugal) – Mindelo (Cape Verde Islands)

The research cruise MSM10/1 was extremely successful. All programs were able to collect high quality data and the anticipated goals of the expedition were fully met. We have been able to carry out the first comprehensive survey of a tracer release in the Guinea Upwelling region (GUTRE) roughly seven month after the tracer was released at 8°N 23°W in April 2008. We have estimated that a total of 40% of the tracer was found during this cruise. While the horizontal spreading and mixing was larger than anticipated, the vertical extent of the tracer found was small. The low vertical tracer spreading rate estimates are supported by the micro structure profile data. The extensive survey of the upper 1000m of the oxygen minimum zone (OMZ) allowed comparing our sections with several previous surveys. We found that the lowest oxygen values in the core of the OMZ have dropped at record low values below 40 μmol/kg. The preliminary findings from the trace metal work focused on Fe ligand measurements shows a slight higher excess ligand concentration in the surface (50m) for three stations. The two other stations show a slight decrease at this depth. A large number of biochemical samples were taken and were analyzed in Kiel for DNA and RNA diversity. The tracer release experiment provided an ideal environment for repeated biochemical sampling in the same water mass

On the role of circulation and mixing in the ventilation of oxygen minimum zones with a focus on the eastern tropical North Atlantic

Ocean observations carried out in the framework of the Collaborative Research Center 754 (SFB 754) "Climate-Biogeochemistry Interactions in the Tropical Ocean" are used to study (1) the structure of tropical oxygen minimum zones (OMZs), (2) the processes that contribute to the oxygen budget, and (3) long-term changes in the oxygen distribution. The OMZ of the eastern tropical North Atlantic (ETNA), located between the well-ventilated subtropical gyre and the equatorial oxygen maximum, is composed of a deep OMZ at about 400 m depth with its core region centred at about 20° W, 10° N and a shallow OMZ at about 100 m depth with lowest oxygen concentrations in proximity to the coastal upwelling region off Mauritania and Senegal. The oxygen budget of the deep OMZ is given by oxygen consumption mainly balanced by the oxygen supply due to meridional eddy fluxes (about 60%) and vertical mixing (about 20%, locally up to 30%). Advection by zonal jets is crucial for the establishment of the equatorial oxygen maximum. In the latitude range of the deep OMZ, it dominates the oxygen supply in the upper 300 to 400 m and generates the intermediate oxygen maximum between deep and shallow OMZs. Water mass ages from transient tracers indicate substantially older water masses in the core of the deep OMZ (about 120–180 years) compared to regions north and south of it. The deoxygenation of the ETNA OMZ during recent decades suggests a substantial imbalance in the oxygen budget: about 10% of the oxygen consumption during that period was not balanced by ventilation. Long-term oxygen observations show variability on interannual, decadal and multidecadal time scales that can partly be attributed to circulation changes. In comparison to the ETNA OMZ the eastern tropical South Pacific OMZ shows a similar structure including an equatorial oxygen maximum driven by zonal advection, but overall much lower oxygen concentrations approaching zero in extended regions. As the shape of the OMZs is set by ocean circulation, the widespread misrepresentation of the intermediate circulation in ocean circulation models substantially contributes to their oxygen bias, which might have significant impacts on predictions of future oxygen levels

Oxygen Supply to the Tropical North East Atlantic Oxygen Minimum Zone

Diapycnal and meridional oxygen supply pathways into the Tropical North East Atlantic Oxygen Minimum Zone (TNEA OMZ) are quantified using a comprehensive observational data set of microstructure measurements, moored observations and repeat ship sections that were carried out in the TNEA during the recent decade. Referring to a state-of-the-art estimate of oxygen consumption, an oxygen budget for the TNEA OMZ is derived. At the OMZ core depth, diapycnal mixing and meridional eddy mixing can account for most of the oxygen supply that is needed to balance the oxygen consumption. In contrast, above the OMZ core depth (100m – 400m) most of the oxygen supply remains unexplained, which is likely balanced by mean zonal advection

Hydrography in the Northern Tropical Atlantic at 1026.8 - 1027.3 kg/m'3 2,1 25 Density Range During 1950 – 2009

The study investigates salinity, depth and thickness of the isopycnals long term changes in the north northern tropical Atlantic through the density range 1026.8:1027.3 kg m-3, where oxygen minimum zone is situated. The salinity front is situated at around l8° N in the chosen density range and hydrographic property evolution is found being different to the north and south domains of the salinity front. Higher salinities during 70s than 2000-2009 period are estimated in the northern domain for all isoclines, but lower in the southern domain. Strong increase in density layer depths is observed in the southern domain intermediate water masses, i.e. Antarctic Intermediate water masses. The strongest correlation between depth and salinity anomalies is found to be -0.5 for the Antarctic Intermediate water masses, being about twice !arger than the estimates for any other water mass. In the end, the salinity front parameters such as width, gradient and variability are described over entire north tropical Atlantic